Protein and nitrogen (N) requirements of infants and the optimum protein level of infant formulas are still under debate. Human milk (HM) N is considered adequate although 25% of the N is provided as non-protein nitrogen (NPN), half in the form of urea. Soluble N, insoluble N, NPN and urea N were determined in HM and cow's milk (CM), 5 CM-based infant formulas, 4 soy protein-based formulas (SF), and manufacturers' skim milk and whey protein sources. Total, peptide and free amino acids were also determined in all samples. Levels of NPN and urea N in formulas were highly dependent on the type of whey used, with ion-exchange whey being highest, followed by electrodialyzed and ultrafiltered whey, respectively. SF contained very little NPN with the exception of Soyalac. Consequently, true protein [(Total N-NPN) x 6.38] was lower than the reported levels in several formulas. Levels of peptide and free amino acids were also affected by the method of whey preparation. While it has been recognized that "casein-predominant" CM formulas will have different effects on metabolic indices than "whey-predominant" formulas, this study demonstrates that there are also pronounced differences among "whey-predominant" formulas.